That's due to implicit conversion and whether or not it results in an index not being properly used has to do with data type precedence. If the index is on something lower on the list (such as varchar) and you pass in something higher (such as int) then SQL is going to convert the value for every row from varchar to int before doing the comparison resulting in a scan. If it's the other way around (index on int and you're passing in varchar) then the implicit conversion will happen on the parameter and you can still get a seek. However, it is best to make sure the data types match so you don't need to worry about implicit conversions at all.